1. Field of the Invention
The present invention relates to a lead-based alloy for a lead-acid battery, a grid for a lead-acid battery, and a lead-acid battery, particularly, to a lead-based alloy for a lead-acid battery, a grid for a lead-acid battery, and a lead-acid battery, which exhibit excellent corrosion resistance and excellent mechanical strength when used for a grid and which are adapted for gravity casting, continuous casting and a rolling process.
2. Description of the Related Art
In recent years, the temperature within the engine compartment of a vehicle is markedly elevated because of the increase in the facilities and the design that eliminates an extra space. Therefore, the lead-acid battery mounted in the vehicle is kept overcharged and operated under severe conditions. As a result, corrosion of the grid and deformation by elongation of the grid, which is called growth, are generated in the positive plate included in the lead-acid battery so as to shorten the life of the lead-acid battery. Naturally, it is of high importance to overcome these problems.
To be more specific, it is necessary for the grid of the lead-acid battery to have a reasonable mechanical strength in order to avoid the deformation during the manufacturing process of the lead-acid battery. It should be noted in this connection that the deformation by elongation, i.e., the generation of the so-called “growth”, is derived from the elongation stress generated by the corroded material formed during use of the battery. The generation of the growth is a problem that is more serious than the problem generated by insufficient mechanical strength. The growth tends to impair the electrical connection between the grid and the active material so as to give rise to serious defects such as a decrease in the battery capacity and short-circuiting caused by the deformation of the plate.
It is possible to lower the growth to some extent by improving the mechanical strength of the grid. However, the growth is a phenomenon that takes place as a result of the mutual function between the mechanical strength of the grid and the corrosion of the grid. Therefore, the effect of improving the mechanical strength of the grid is rendered limitative unless the corrosion is lowered.
It should also be noted that the lead-acid battery is strongly required to be free from maintenance in view of convenience in handling. It was customary for the grid constituting the positive electrode of the lead-acid battery to be formed of a lead-based alloy comprising 0.06 to 0.10% by weight of Ca, 1.0 to 2.0% by weight of Sn, 0.005 to 0.04 by weight of Al, and the balance of Pb. However, it was impossible for even the grid made of the lead-based alloy of the particular composition to produce a sufficient effect of improving the corrosion resistance and suppressing the growth, resulting in failure to overcome the problem in terms of the life of the lead-acid battery.
Several ideas have already been proposed in an attempt to overcome the difficulty. For example, proposed in WO-97/30183 is a grid for a lead-acid battery made of a lead-based alloy comprising 0.05 to 0.12 by weight of Ca, not more than 3% by weight of Sn, 0.002 to 0.04% by weight of Al, and not more than 0.02 by weight of Ba. It is reported that the grid for the lead-acid battery of the particular composition is capable of maintaining a reasonable mechanical strength over the entire life of the battery. Proposed in U.S. Pat. No. 4,233,070 is a grid for a lead-acid battery. It is reported that the mechanical strength of the grid for the lead-acid battery can be improved without impairing the corrosion resistance by adding 0.005 to 0.05% by weight of Mg to a lead-based alloy comprising alkaline earth metals such as Ca, Sr and Ba as well as Sn and Al. Proposed in U.S. Pat. No. 4,358,518 is a grid for a lead-acid battery made of an alloy comprising 0.03 to 0.04% by weight of Ca, 0.15 to 0.4% by weight of Sr, 0.15 to 0.9% by weight of Sn, and 0.025 to 0.07% by weight of Ba. It is reported that the grid for the lead-acid battery made of the alloy of the particular composition permits improving both the mechanical strength and the corrosion resistance of the grid. Further, proposed in U.S. Pat. No. 5,298,350 is a lead alloy, which exhibits a long life under high temperatures, comprising 0.025 to 0.06% by weight of Ca, 0.3 to 0.7% by weight of Sn, and 0.015 to 0.045% by weight of Ag.
As pointed out above, the U.S. patents quoted above teach that Ba contained in the lead-based alloy for the lead-acid battery is effective in improving the mechanical strength of the alloy. Also, WO-97/30183 quoted above teaches that prescribed amounts of Ca and Ba are effective for achieving the mechanical strength required for the grid of the positive electrode included in the battery.
However, any of the prior arts quoted above, which are directed to the alloy containing Ba, is insufficient for improving both the mechanical strength and the corrosion resistance of the grid. This is also the case with U.S. Pat. No. 5,298,350 quoted above, which teaches the addition of Ag. To be more specific, the growth is certainly suppressed in this prior art in accordance with some improvement in the corrosion resistance and the mechanical strength. However, the brittleness is also increased in this prior art so as to give rise to a decisive defect such as breakage of the grid in some cases. Such being the situation, it was difficult to obtain a lead-acid battery of a long life.
As described above, the conventional lead-based alloy used for the manufacture of the grid for a lead-acid battery was incapable of withstanding sufficiently stably operation for a long time under high temperature.
On the other hand, the battery for a vehicle is strongly required to be rendered lightweight in view of fuel consumption and saving natural resources. In order to decrease the thickness of the grid in an attempt to meet the particular requirement, it is necessary to improve both the corrosion resistance and the mechanical strength of the grid at a high level.
Further, in accordance with increase in the battery voltage for a vehicle from 12 V to 36 V and with the popularization of a hybrid vehicle (HEV), it is of high emergency to further promote the environmental improvement and the improvement in the fuel consumption. Under such new application, it is necessary to achieve a high current charge and discharge under high temperatures, and the long life achieved by the improvement in the corrosion resistance is further required. To decrease the thickness of the plate for increasing the specific surface area of the plate is effective for improving the high current charge-discharge characteristics. However, the problems in respect of the corrosion resistance and the growth generation are rendered more prominent if the thickness of the plate is decreased.
Incidentally, the problems and the subject matters described above are not limited to the battery for a vehicle. Problems and subject matters substantially equal to those described above are also inherent in the battery for a back up power source and the battery for energy storage, which are widely used in, for example, IT.